In regards to the small, black box transformer-rectifiers one sees all the time on things like printers, speakers, battery chargers etc... for computers and other appliances, how tight are they on specifications for output? For example, if I have a device that is rated 12V DC and 4 amps, how sensitive are the appliances in reference to that rating? Do they have a standard spec for things like ripple? The reason I am asking is that 1) I have always been curious about this and 2) I have come across a need to find a replacement that I can not get from the OEM. So my device states the rating of 12VDC and 4 amps. If I were to track down a similar unit at Radio Shack, do I stand a chance of damaging the unit? If anyone can give a nice little primer on these transformer rectifiers, I would be much appreciative.